What it is and what it does
The Siemens 5SU9356-7KP20 is a 2-pole (1P+N) RCD-operated circuit breaker from the SENTRON 5SU9 series — an RCBO that combines overcurrent protection and residual current detection in a single 36 mm wide module. Rated 20 A at AC with a C tripping characteristic, it handles resistive and moderate motor-start loads on lighting, socket, or small-machine circuits. The Type A residual-current element catches pulsating DC faults (common with VFDs, LED drivers, and switched-mode supplies) as well as sinusoidal AC faults. Breaking capacity is 6 kA per EN 60898 and 10 kA per IEC 60947-2, so it's rated for domestic and light commercial distribution boards where the prospective fault current stays under those limits.
Ratings and what they mean for fit
The 20 A rating is at 30 °C ambient; at 40 °C it derates to 18.8 A, at 45 °C to 18.2 A, at 50 °C to 17.4 A, and at 55 °C to 16.2 A. If the panel runs warm — say 45 °C inside a sealed enclosure — the breaker's effective capacity drops to 18.2 A, so size the load accordingly. The C-curve (magnetic trip at 5–10× In) means it's suited for circuits with moderate inrush, like small motors or transformers, without nuisance tripping. The Type A residual-current detection covers pulsed DC faults up to 6 mA superimposed on AC, which is the standard for equipment with single-phase rectifiers. The unit is halogen-free and silicon-free, which matters for clean-room or high-reliability environments where outgassing can't be tolerated.
Mounting and integration
The 5SU9356-7KP20 snaps onto a standard 35 mm DIN rail. Width is 36 mm (2 width units), depth 77 mm, installation depth 70 mm — it fits in any distribution board with that clearance. Mounting position is any orientation, so no restrictions on vertical or horizontal rail placement. Supply can be fed from either top or bottom, which simplifies busbar routing in crowded enclosures. Terminal capacity accepts solid or stranded conductors from 0.75 mm² up to 35 mm², covering most downstream circuit sizes. IP20 rating is standard for installed distribution boards; the breaker itself is touch-protected per the product feature.